Nearest-neighbor Interactions Research Articles

Hilbert Space Shattering and Disorder-Free Localization in Polar Lattice Gases.

Emerging dynamical constraints resulting from intersite interactions severely limit particle mobility in polar lattice gases. Whereas in absence of disorder hard-core Hubbard models with only strong nearest-neighbor interactions present Hilbert space fragmentation but no many-body localization for typical states, the 1/r^{3} tail of the dipolar interaction results in Hilbert space shattering, as well as in a dramatically slowed down dynamics and eventual disorder-free localization. Our results show that the study of the intriguing interplay between disorder- and interaction-induced many-body localization is within reach of future experiments with magnetic atoms and polar molecules.

THE DYNAMICS OF A PARTICLE IN A TIGHT-BINDING LATTICE UNDER A COULOMB POTENTIAL

We study the tight-binding dynamics of a charged particle by the hopping mechanism in a one - dimensional lattice under the action of a Coulomb potential due to another fixed particle (impurity). The quantum dynamics is studied using the pseudo-spectral method applied to a tight-binding Hamiltonian with nearest neighbors interactions. The resulting expected values for position and velocity are then compared with those deduced by the semi-classical method invoking the effective potential in the lattice. These results are notably similar when the particle is located far from the impurity position since the Coulomb field is approximately uniform, while when near to the impurity the results are different due to the quantum dispersion of the particle’s wavepacket under a highly inhomogeneous field. Interestingly, however, the quantum and semiclassical results coincide in the continuous regime (parabolic dispersion relation) due to the transmission of the particle’s wavepacket through the impurity.

Synthesis and characterization of wide band excited yellow phosphor LaNb2VO9:Dy3+ and application in potential fingerprint and lip print detection

LaNb2VO9:Dy3+ yellow phosphors were synthesized by high-temperature solid-state method. The luminescent properties, structure, and color purity of the products were studied systematically. LaNb2VO9:Dy3+ phosphors can be effectively excited by two broad charge transfer bands (CTB) under 307 nm excitation. The strong yellow light at 570 nm was caused by the 4F9/2 → 6H13/2 energy level transition of Dy3+. The optimum doping concentration of LaNb2VO9:xDy3+ phosphor was x = 0.10 mol, and the dominant factor of concentration quenching effect is the nearest-neighbor ions interaction. The chromaticity coordinates of International Committee LaNb2VO9:Dy3+ were located in the yellow color area. In addition, LaNb2VO9:0.1Dy3+ phosphor can well visualize the latent fingerprint (LFP) image and lip print image. The levels 1–3 features of fingerprint and groove pattern of lip print can be easily recognized, with high resolution and contrast. The experimental results showed LaNb2VO9:Dy3+ can be well applied to LFP and lip print detection.

Giant Biquadratic Exchange in 2D Magnets and Its Role in Stabilizing Ferromagnetism of NiCl2 Monolayers

Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultra-compact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.

Dynamic polymers: invariant measures and ordering by noise

We develop a dynamical approach to infinite volume directed polymer measures in random environments. We define polymer dynamics in 1+1 dimension as a stochastic gradient flow on polymers pinned at the origin, for energy involving quadratic nearest neighbor interaction and local interaction with random environment. We prove existence and uniqueness of the solution, continuity of the flow, the order-preserving property with respect to the coordinatewise partial order, and the invariance of the asymptotic slope. We establish ordering by noise which means that if two initial conditions have distinct slopes, then the associated solutions eventually get ordered coordinatewise. This, along with the shear-invariance property and existing results on static infinite volume polymer measures, allows to prove that for a fixed asymptotic slope and almost every realization of the environment, the polymer dynamics has a unique invariant distribution given by a unique infinite volume polymer measure, and, moreover, One Force -- One Solution principle holds. We also prove that every polymer measure is concentrated on paths with well-defined asymptotic slopes and give an estimate on deviations from straight lines.

Phase inhomogeneity and inapplicability of Jeff=0 description in dilute Ir5+ doped Ba2YSb1−xIrxO6 (x=0.1, 0.2, 0.3)

Various exotic magnetic ground states have been anticipated in the higher transition metal $(4d \mathrm{and} 5d)$ oxides with large atomic spin-orbit coupling (SOC). However, many such expectations were not met because the exact atomic SOC strength and its consequent influence on electronic and magnetic ground states were always masked by solid-state effects. Here, we attempt to dope a dilute amount of ${\mathrm{Ir}}^{5+}$ $(x=0.1--0.3)$ in place of ${\mathrm{Sb}}^{5+}$ in perfectly cubic ${\mathrm{Ba}}_{2}{\mathrm{YSbO}}_{6}$ with a target to minimize the effects of hopping between distant Ir ions and noncubic crystal field effect to promote the effect of SOC and to realize the coveted ${J}_{\text{eff}}=0$ ground state, expected for $jj$-coupled $5{d}^{4}\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{5+}$ systems, purely from the consideration of atomic SOC. However, despite certain inhomogeneity in the distribution of the dopant ${\mathrm{Ir}}^{5+}$ ions in the system, our experimental results reveal high magnetic moments (away from the ${J}_{\text{eff}}$ = 0 picture) with strong nearest-neighbor interaction, but without any long-range ordering, for the lowest doped system which continues to decrease with increasing Ir content, completely contrary to the expectation of the $jj$-coupling description. This observation strongly indicates that presence of a localized Ir moment, originating from $L\text{\ensuremath{-}}S$ coupling interaction, might be the most appropriate mechanism here.

Effects of composition variation and site disorder on the magnetic interactions in Ru2Fe(Si1-xGex) alloys

X-ray diffraction studies on arc melted Ru2Fe(Si1-xGex) (0 ≤ x ≤ 1) alloys did not show the characteristic (111) and (200) super-lattice peaks of the stable L21 structure of full Heusler alloys. The alloy with x = 0 exhibited antiferromagnetic behaviour. However, with increase in x, the alloys became ferromagnetic. In order to understand the origin of the novel magnetic behavior of these alloys and its correlation with their crystal structure, ab initio study was carried out using spin-polarized relativistic Korringa-Kohn-Rostoker Green’s function method. The study shows that the antiferomagnetism in the alloy with x = 0 is due to the presence of B2 disorder. Due to this, the 2nd nearest neighbour (NN) exchange interaction between the Fe atoms at proper and improper sites, respectively, becomes strongly antiferromagnetic which dominates over the 1st NN ferromagnetic exchange interaction. With increase in x, the alloys become ferromagnetic with an improvement in L21 ordering which in turn makes the 1st NN interaction strongly ferromagnetic. The electronic structure calculations show no half metallic gap in either spin direction for all the alloys. Comparison of experimental and theoretically estimated properties of Ru2Fe(Si1-xGex) alloys provides insight to the variation in the magnetic interaction with the composition and the disorder present in the alloys.

Required nearest-neighbor Coulomb interactions for a charge-ordered phase transition in (TMTTF)2MF6 with inversion symmetry breaking in crystal

We examined the charge-ordered phase transition in (TMTTF)2MF6 systems by exact diagonalization of an extended Hubbard model Hamiltonian whose parameters were referred from quantum chemical calculations with periodic boundary conditions along with inversion symmetry breaking in the crystal. We showed that a nearest-neighbor Coulomb interactions for the charge-ordered phase transition should be twice as much as those previously obtained by fitting for conductivities with a minute inversion symmetry breaking of the crystal (∼0.01 Å) which was confirmed by a recent experimental study.

Quantum computing for classical problems: variational quantum eigensolver for activated processes

The theory of stochastic processes impacts both physical and social sciences. At the molecular scale, stochastic dynamics is ubiquitous because of thermal fluctuations. The Fokker–Plank–Smoluchowski equation models the time evolution of the probability density of selected degrees of freedom in the diffusive regime and it is, therefore, a workhorse of physical chemistry. In this paper we report on the development and implementation of a variational quantum eigensolver to solve the Fokker–Planck–Smoluchowski eigenvalue problem. We show that such an algorithm, typically adopted to address quantum chemistry problems, can be effectively applied to classical systems, paving the way to new applications of quantum computers. We compute the conformational transition rate in a linear chain of rotors with nearest-neighbour interactions. We provide a method to encode the probability distribution for a given conformation of the chain on a quantum computer and assess its scalability in terms of operations. A performance analysis on noisy quantum emulators and quantum devices (IBMQ Santiago) is provided for a small chain which shows results in good agreement with the classical benchmark without any further addition of error mitigation techniques.

The role of epitaxial strain on the electronic and magnetic structure of La0.7Sr0.3MnO3/LaCoO3 bilayers

Realizing atomically flat interfaces between the ultrathin perovskite oxides is a challenging task, which usually possess different chemical environments, depending on the terminating lattice planes. Hence, tuning the interfaces across the heterostructures for desired electrical and magnetic properties is a powerful approach in oxide electronics. Focusing on these aspects, in the present work we employ a novel strategy of engineering the interfaces through the layer stacking sequence and degree of strain to probe the changes occurring in the local atomic environment at the interfaces, magnetic behaviour, and electronic properties of ferromagnetic bilayers La0.7Sr0.3MnO3 (LSMO)/LaCoO3 (LCO) grown by the pulsed laser deposition technique. The biaxial tensile strain experienced by these layers drives the ferromagnetic (FM) ordering temperatures to lower values as compared to their bulk counterparts. Interestingly, the bilayer sequence LCO (15 nm)/LSMO (5 nm) (BL2) exhibits large magnetocrystalline anisotropy (Ku ≈ 4.7 × 104 erg/cc) and weak anti-FM coupling across the interface of the two FM constituents, resulting in a partial compensation in the magnetic moment of the system within a specific temperature window (ΔT = 184 − 82 K). However, for T ≤ 82 K, the FM superexchange interaction between the trivalent Co high-spin and low-spin states dominates the overall magnetic ordering in BL2. The magnetodynamic features probed by the frequency dependent FM resonance (FMR) on this system yield the gyromagnetic ratio (γ/2π ∼ 29.22 GHz/T), demagnetization fields (4πMeff ∼ 3770 Oe), and effective damping constant (αeff ∼ 0.0143) for the BL2 configuration. Moreover, the strength of the nearest-neighbor exchange interaction Jeff in the BL2 configuration exhibits linear falloff with the increasing LCO layer thickness (2 nm ≤tLCO≤ 18 nm). This scenario is also consistent with the variation of the effective number of spins available per unit volume [10 cm−3 ≤ NV(×1022) ≤ 2 cm−3] with increasing tLCO. As tLCO approaches negligibly small values (<2 nm), the magnitude of Jeff/kB reaches its maximum ∼5.47 K (for LCO) and 21.93 K (for LSMO), which is in good agreement with Jeff/kB ∼ 5 ± 2 K (20 ± 2 K) for highly epitaxial LCO (LSMO) single layers. These results demonstrate that the layer sequence control of magnetic coupling across the interfaces opens a constructive approach for exploring the novel electronic devices.

Existence of exponentially spatially localized breather solutions for lattices of nonlinearly coupled particles: Schauder’s fixed point theorem approach

The problem of showing the existence of localized modes in nonlinear lattices has attracted considerable efforts not only from the physical but also from the mathematical viewpoint where a rich variety of methods have been employed. In this paper, we prove that a fixed point theory approach based on the celebrated Schauder’s fixed point theorem may provide a general method to concisely establish not only the existence of localized structures but also a required rate of spatial localization. As a case study, we consider lattices of coupled particles with a nonlinear nearest neighbor interaction and prove the existence of exponentially spatially localized breathers exhibiting either even-parity or odd-parity symmetry under necessary non-resonant conditions accompanied with the proof of energy bounds of solutions.

Heisenberg spins on an anisotropic triangular lattice: PdCrO2 under uniaxial stress

When Heisenberg spins interact antiferromagnetically on a triangular lattice and nearest-neighbor interactions dominate, the ground state is 120° antiferromagnetism. In this work, we probe the response of this state to lifting the triangular symmetry, through investigation of the triangular antiferromagnet PdCrO2 under uniaxial stress by neutron diffraction and resistivity measurements. The periodicity of the magnetic order is found to change rapidly with applied stress; the rate of change indicates that the magnetic anisotropy is roughly forty times the stress-induced bond length anisotropy. At low stress, the incommensuration period becomes extremely long, on the order of 1000 lattice spacings; no locking of the magnetism to commensurate periodicity is detected. Separately, the magnetic structure is found to undergo a first-order transition at a compressive stress of ∼0.4 GPa, at which the interlayer ordering switches from a double-to a single-q structure.

Nontrivial damping of quantum many-body dynamics

Understanding how the dynamics of a given quantum system with many degrees of freedom is altered by the presence of a generic perturbation is a notoriously difficult question. Recent works predict that, in the overwhelming majority of cases, the unperturbed dynamics is just damped by a simple function, e.g., exponentially as expected from Fermi's golden rule. While these predictions rely on random-matrix arguments and typicality, they can only be verified for a specific physical situation by comparing to the actual solution or measurement. Crucially, it also remains unclear how frequent and under which conditions counterexamples to the typical behavior occur. In this work, we discuss this question from the perspective of projection-operator techniques, where exponential damping of a density matrix occurs in the interaction picture but not necessarily in the Schrödinger picture. We show that a nontrivial damping in the Schrödinger picture can emerge if the dynamics in the unperturbed system possesses rich features, for instance due to the presence of strong interactions. This suggestion has consequences for the time dependence of correlation functions. We substantiate our theoretical arguments by large-scale numerical simulations of charge transport in the extended Fermi-Hubbard chain, where the nearest-neighbor interactions are treated as a perturbation to the integrable reference system.

On the precise value of the magnetic ordering temperature

For many magnets with a three-dimensional spin, experimental data of the magnetic ordering temperature, Tc, and of the magnon energy at the zone boundary, EZB (the near-neighbor interaction strength) are compared. It is observed that the lower the spin quantum number is, the lower is Tc in comparison with EZB/kB. This is explained by the fact that the magnetic ordering transition is the ordering transition of a boson field and occurs largely decoupled from the exchange interactions between the spins. Due to a strong interaction with the bosons, the spins order at the same temperature as the boson field. The bosons are essentially magnetic dipole radiation, emitted through stimulated emission by the precessing spins. Surprisingly, in the critical range (and below), the bosons dominate the dynamics of the spins totally, and prevent the local exchange interactions from performing a phase transition into a long-range ordered state. Condition for the ordering transition of the boson field is that the density of the bosons, travelling along those crystallographic directions along which domains will be formed below Tc, is sufficiently high, such that the threshold for the spontaneous onset of stimulated emission is reached. In the ordered state of the boson field, the bosons are in a one-dimensional and coherent state. This is realized in each magnetic domain. Since in magnets with a low spin quantum number the bosons get generated with a low rate, the critical density, necessary for the ordering transition of the boson field is reached at a correspondingly low temperature only.

Simulated effects of sample size and grain neighborhood on the modeling of extreme value fatigue response

Assessing the size of representative volume elements (RVEs) for fatigue-related applications is challenging. A RVE relevant to random microstructure requires a volume of material that is sufficiently large to capture the grain/phase heterogeneity that captures all statistical moments of the distribution of the driving force for fatigue crack formation at “hot spot” grains. Consequently, the large size of a microstructure RVE required to study fatigue phenomena is largely computationally intractable and difficult to explore. A more realistic objective in this work is to systematically study, as a function of the size of a statistical sample of microstructure, trends towards convergence of the simulated distribution of driving force for fatigue crack formation. The present work accordingly leverages the recently developed open-source PRISMS-Fatigue framework [Yaghoobi et al., npj Comput. Mater., 7, 38 (2021)] to examine the trends in convergence of extreme value distributions (EVD) of Fatigue Indicator Parameters (FIPs) in progressively larger polycrystalline microstructure realizations of FCC Al alloy 7075-T6 using crystal plasticity finite element method simulations. The results are compared to the traditional method in which ensembles of statistical volume elements (SVEs) are simulated to build up statistics intended to approximate those associated with a larger volume of material. The convergence of EVDs with increase of size of a SVE of microstructure is closely related to the extent of grain nearest neighbor (NN) interactions. Accordingly, the sensitivity of the local micromechanical response at hot spot grains is quantitatively investigated by systematically varying the orientations of NN grains. Results indicate that SVEs with cubic crystallographic texture tend towards convergence of the EVD of FIPs with tens of thousands of grains while the random and rolled textures require larger volumes. Simple relationships based on microstructure parameters (e.g., Schmid Factor, grain size, NN misorientation) do not completely correlate to fatigue hot spot grains. Finally, the sensitivity of the extreme value fatigue response at hot spot grains extends to the 3rd NN when a single neighborhood grain orientation is altered.

Observation of interaction-induced phenomena of relativistic quantum mechanics

Relativistic quantum mechanics has been developed for nearly a century to characterize the high-energy physics in quantum domain, and various intriguing phenomena without low-energy counterparts have been revealed. Recently, with the discovery of Dirac cone in graphene, quantum materials and their classical analogies provide the second approach to exhibit the relativistic wave equation, making large amounts of theoretical predications become reality in the lab. Here, we experimentally demonstrate a third way to get into the relativistic physics. Based on the extended one-dimensional Bose-Hubbard model, we show that two strongly correlated bosons can exhibit Dirac-like phenomena, including the Zitterbewegung and Klein tunneling, in the presence of giant on-site and nearest-neighbor interactions. By mapping eigenstates of two correlated bosons to modes of designed circuit lattices, the interaction-induced Zitterbewegung and Klein tunneling are verified by measuring the voltage dynamics. Our finding not only demonstrates a way to exhibit the relativistic physics, but also provides a flexible platform to further investigate many interesting phenomena related to the particle interaction in experiments.

Weighted Bures length uncovers quantum state sensitivity.

The unitarity of quantum evolutions implies that an overlap between two initial states does not change in time. This property is commonly believed to explain the apparent lack of state sensitivity in quantum theory, a feature that is prevailing in classical chaotic systems. However, classical state sensitivity is based on a distance between two trajectories in phase space which is a completely different mathematical concept than an overlap between two vectors in Hilbert space. It is possible that state sensitivity in quantum theory can be detected with the help of some special metric. Here we show that the recently introduced Weighted Bures length achieves this task. We numerically investigate a unitary cellular automaton of N interacting qubits and analyze how a single-qubit perturbation affects the evolution of WBL between the unperturbed and perturbed states. We observe a linear growth of WBL if the qubits are arranged into a cyclic graph and an exponential growth if they are arranged into a random bipartite graph.

Observing two-particle Anderson localization in linear disordered photonic lattices.

We theoretically and systematically investigate Anderson localization of two bosons with nearest-neighbor interaction in one dimension under short- and long-time scales, two types of disorders, and three types of initial states, which can be directly observed in linear disordered photonic lattices via two experimentally measurable physical quantities, participation ratio and spatial correlation. We find that the behavior of localization characterized by the participation ratio depends on the strength of interaction and the type of disorder and initial condition. Two-boson spatial correlation reveals more novel and unique features. In the ordered case, two types of two-boson bindings and bosonic "fermionization" are shown, which are intimately attributed to the band structure of the system. In the disordered case, the impact of interaction on the two-boson Anderson localization is reexamined and the joint effect of disorder and interaction is addressed. We further demonstrate that the independence of the participation ratio or spatial correlation on the sign of interaction can be eliminated by employing an initial state that breaks one of two specific symmetries. Finally, we elucidate the relevant details of the experimental implementation in a two-dimensional linear photonic lattice.

Static and dynamic magnetic properties of honeycomb lattice antiferromagnets Na2M2TeO6 , M=Co and Ni

The magnetic structures and spin dynamics of Na$_{2}$Co$_{2}$TeO$_{6}$ and Na$_{2}$Ni$_{2}$TeO$_{6}$ are investigated by means of elastic and inelastic neutron scattering measurements and the results are discussed in the context of a generalized Kitaev-Heisenberg model on honeycomb lattice with strong spin-orbit coupling. The large number of parameters involved in the Hamiltonian model are evaluated by using an iterative optimization algorithm capable of extracting model solutions and simultaneously estimating their uncertainty. The analyses establish that both Co$^{2+}$ ($d^7$) and Ni$^{2+}$ ($d^8$) antiferromagnets realize bond-dependent anisotropic nearest-neighbor interactions, and support the theoretical predictions for the realization of Kitaev physics in 3$d$ electron systems with effective spins $S$=1/2 and $S$=1. Furthermore, by studying the Na-doped system Na$_{2.4}$Ni$_{2}$TeO$_{6}$, we show that the control of Na content can provide an effective route for fine tuning the magnetic lattice dimensionality, as well as to controlling the bond-dependent anisotropic interactions.

Antiferromagnetic Kitaev interaction inJ eff = 1/2 cobalt honeycomb materials Na3Co2SbO6 and Na2Co2TeO6

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue for quantum magnetism research. We conclude that Na3Co2SbO6 and Na2Co2TeO6 are new honeycomb cobalt-based systems with AFM Kitaev interaction by carrying out inelastic neutron scattering experiments and subsequent analysis. The spin–orbit excitons observed at 20–28 meV in both compounds strongly support the idea that Co2+ ions of both compounds have a spin-orbital entangled J eff = 1/2 state. Furthermore, we found that a generalized Kitaev–Heisenberg Hamiltonian can describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show significant AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We also found a strong magnon-damping effect at the higher energy part of the spin waves, entirely consistent with observations in other Kitaev magnets. Our work suggests Na3Co2SbO6 and Na2Co2TeO6 as rare examples of the AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.